Compression Test

The pistons separate the combustion chamber with the use of 3 rings. Each has a different use but when they wear down you can now push gases from the combustion chamber into the crank case which both increases your PCV flow and lowers the compression in the cylinder. You will also get oil in your cylinders in vacuum conditions which can cause you to smoke and consume large quantities of oil. Neither is good. This is a write up for the turbo so you NA people will differ a bit, but either way you should check the TSRM Page first, then consult my "in the field" way and choose which you like best.

Tools:		Ratchet		8" extension
		10mm Socket		U-Joint
		12mm Socket		Torque Wrench
		12mm Long Socket		Compression Gauge
		16mm Long Socket
Materials:		Anti-Seize goo
		A bit of oil

1.	Remove the two 10mm bolts holding down the ISC pipe and the remove it by disconnecting the two vacuum hoses and large black hose on either side.
2.	Remove the 5 10mm nuts holding down the ignition pack cover and remove the oil cap. Now carefully remove the ignition pack cover and then replace the oil cap.
3.	Disconnect the CPS connector on top of the CPS.
4.	Disconnect the Solenoid Resistor Connector between the fuse box and strut tower cover.
5.	Using a 12mm long socket remove the 3 nuts holding the throttle body linkage assembly to the exhaust side of the head. Remove the throttle body linkage arm and remove the 12mm bolt holding the throttle body linkage assembly to the center valley. Now you can maneuver this out of your way when you need to.
6.	Disconnect the ignition pack connector, the 6 spark plug wires and using a 12mm wrench remove the bolt holding the grounding wire to the intake valve cover. Now remove the ignition pack from the car.
7.	Carefully remove the 6 sparkplug wires.
8.	If you have compressed air now is a good time to blow out the spark plug wells to get the crud and oil out before you remove the plugs.
9.	Using a 16mm long socket and an extension remove the 6 plugs. Plug #4 will be difficult because the PCV pipe is in the way. so attach your universal joint to that socket then you extension to that and it should work fine. When you take the plugs out keep them in numbered order.
10.	Now hook your gauge up to one cylinder. Make sure both connections are secure and the gauge is in a safe place, or have a helper hold it, and crank your engine 4-5 times. You should crank it until the needle no longer moves. I did 4 times each and got about 165, when I did 5 I got 170. So I should have tried more, but I am lazy. You also don't want to drain your battery cus that will effect your test, but I wasn't showing any signs of weakness after I finished my dry test. I didn't do the wet test so I don't know what repeating it would do to my battery strength.
11.	Repeat for each cylinder writing down the results after each test.
12.	If one or more of the cylinders are "low" then pour 1 Tbls of oil in to the cylinder, let it sit for 2 minutes then repeat the test. This is called the wet test. If your results improve then your rings are bad.

My results we as follows with extra data for the record.

This test was done "soon", 30 minutes, after I stopped and with all the plugs removed. The weather conditions were 29.87inHG, 66.2F, and 79% Humidity. I did the test with 4 cranks though when I did 5 the 165PSI test moved to 170PSI. The head has been shaved once by an unknown amount. I do have oil smoke in the morning which I think is from bad valve stem seals, so that might cause the carbon. I certainly have a lot of oil in the IC piping that could cause it, but low oil consumption (1 qt/3000 miles). I looked at the plugs and they are all a light brownish, a bit darker than normal but nothing extreme.

My car had 126,568 miles on it and the plugs themselves had 27377 miles. It was 6:30PM on 28 September 2002. The oil was also changed a week ago, Mobil 1 Synth, but I doubt that effects anything. The vacuum is a steady at 21.25inHG, needle fluctuating less than a hair. I will be readjusting all the gaps back to 0.030 inch. I didn't install the plugs to begin with, so I can't verify that they were installed with the proper gap, but #2 is really bad at 0.040. No high speed missing or poor idle was noticed so this is just a preventative change. The plug wires are all nicely in spec, though it is quite vague at 25 kOhm maximum.

How to interpret your results
OK, now you have your data points. If they are significantly higher than stated then you could have carbon build-up or your head was shaved increasing your compression. If they are lower and adding oil brought them back up then your rings are probably bad. If the oil didn't do it then you could have a bad valve, or a blown head gasket. If two adjacent cylinders are low the HG is probably blown between them, if just one is low and your coolant isn't doing the over-flowing dance then suspect valves. Either way the next step is a full "leak down test".

Questions, comments, rude remarks contact webmaster. CygnusX1 hits: 5931389

Got Worms?